lint: Enable prefer_relative_imports

See docs:
  https://dart.dev/guides/language/effective-dart/usage#prefer-relative-import-paths

And the rationale for why some consistent convention is needed is
just above that:
  https://dart.dev/guides/language/effective-dart/usage#dont-allow-an-import-path-to-reach-into-or-out-of-lib

Author: gnprice

analysis_options.yaml

@@ -24,6 +24,7 @@ linter:
   rules:
     # avoid_print: false  # Uncomment to disable the `avoid_print` rule
     # prefer_single_quotes: true  # Uncomment to enable the `prefer_single_quotes` rule
+    prefer_relative_imports: true
 
 # Additional information about this file can be found at
 # https://dart.dev/guides/language/analysis-options